The federal government's effort to root out an anonymous critic of President Trump's immigration policies has sparked an investigation into whether officials abused their authority by demanding that Twitter reveal the identity of one of its users, according to a letter released Friday.

The Twitter account was part of an explosion of anonymous online criticism of Trump that began shortly after his inauguration in January and appeared to emanate from within many federal agencies. The Department of Homeland Security's effort to identify the user behind one of these - @ALT_uscis, using the acronym for the U.S. Citizens and Immigration Services - prompted a summons to Twitter demanding that the company reveal the identity of the user, who was suspected to be a federal employee.

The summons, on March 14, provoked a lawsuit from Twitter and sharp criticism from privacy advocates. In the face of this backlash, federal officials rescinded the summons, and the company dropped its suit, appearing to resolve the case.

But DHS Inspector General John Roth decided to pursue an investigation of possible abuse of authority in this case as well as "potential broader misuse of summons authority" within the department, he wrote in a letter Friday to U.S. Sen. Ron Wyden, D-Ore., who had called the effort to unmask the Twitter user a "witch hunt."

Roth said that his office previously had determined that the @ALT_uscis account had not revealed any classified information. The letter also said Roth's office did not play any role in efforts to determine the identity of the Twitter user because of rules intended to "ensure that our work does not have a chilling effort on individuals' free speech rights."

USCIS is part of Customs and Border Patrol, which in turn is part of DHS. They have been on the front lines of Trump's politically charged efforts to limit immigration, a key campaign promise.

The @ALT_uscis account notes on its Twitter page that it does not represent the views of DHS or USCIS but instead provides "immigration resistance & everything else resistance, snark, humor, sarcasm." Its list of followers, which was more than 32,000 before news of the Twitter lawsuit now tops 193,000. It has tweeted more than 10,000 times.

The Supreme Court has ruled that the First Amendment protects the right to speak anonymously or while using a pseudonym, as many of the Founding Fathers did in their own writings. In order to defend its summons to Twitter, DHS officials would have had to demonstrate that the user behind the @ALT_uscis account had likely committed a violation of law.